Giardia lamblia is an intestinal protozoan pathogen common in both the developing world and industrialized nations. It causes an acute diarrhea in most patients with clinical illness, but chronic diarrhea and malabsorption associated with bacterial small bowel overgrowth syndrome occur in some patients, and chronic giardiasis may be an important cause of growth retardation and malnutrition in children in developing nations. Pathogenesis is unknown, thus impeding the development of appropriate new therapeutic or preventative strategies. We have found a lectin activity in the trophozoite of in vitro cultured G. lamblia that mediates attachment to mammalian red blood cells, and which is induced by tryptic activity in human small bowel juice or by trypsin in vitro. To further study the nature of the lectin and its possible role in pathogenesis of giardiasis, we will grow various Giardia species in vitro or, in the case of the mouse pathogen G. muris, in vivo in mice. Lectin activity will be investigated in all strains using a hemagglutination assay. The specificity of binding will be studied by hemagglutination inhibition, using a variety of mono-, di- and complex oligo-saccharides or glycolipids. For initial purification we will use affinity chromatography with immobilized LPS from S. weslaco. This is based on our preliminary data from hapten inhibition studies. Other methods, including isoelectric focusing and ion exchange chromatography will be used as needed. The purified lectin will be used to raise polyclonal and monoclonal antibody in rabbits and mice, respectively. These will be used for comparison of lectins from different Giardia strains and for immunocytochemistry studies to localize lectin on the parasite. The presence and location of receptors on intestinal tissue or in the gut lumen will be studied in the same way with intestinal membrane fractions prepared from mouse tissue. Effects of these inhibitory molecules on parasite growth in vitro and on colonization in vivo will be determined. Thus we expect to fully characterize the lectin of G. lambia and to begin to determine its role in infection.